Time monitoring of packet retransmissions during soft handover

ABSTRACT

The present invention relates to a method for scheduling data retransmissions, a method for use in a data retransmission scheme and a method for updating a soft buffer of a base station in a mobile communication system during a soft-handover. The present invention relates to a base station executing the controlling and updating method, a communication terminal for executing the scheduling method and to a mobile communication system comprising at least one the base station and communication terminal. To prevent erroneous combining of data packets in a packet retransmission scheme at the receiver, the present invention provides a method that may flush the soft bufferregion associated to areceived data packet upon its correct reception. Further, a method is provided that monitors the time elapsed since the last storing of a data packet in a buffer region of a base station to be able to trigger the flush of the buffer region upon expiry of a threshold time period.

The present invention relates to a method for scheduling dataretransmissions and a method for use in a packet retransmission schemein a communication terminal being part of a mobile communication systemcomprising said communication terminal and a plurality of base stations,wherein said communication terminal is in communication with saidplurality of base stations during a soft handover. Moreover the presentinvention relates to a method for updating a soft buffer of a basestation being part of the mobile communication terminal. Further thepresent invention relates to a base station executing the method forcontrolling data retransmissions and a communication terminal executingthe method for scheduling data retransmissions. Finally, the presentinvention relates to a mobile communication system comprising at leastone the base station and at least one communication terminal.

TECHNICAL BACKGROUND

W-CDMA (wideband Code Division Multiple Access) is a radio interface forIMT-2000 (International Mobile Communication), which was standardizedfor use as the 3^(rd) generation wireless mobile telecommunicationsystem. It provides a variety of services such as voice services andmultimedia mobile communication services in a flexible and efficientway. The standardization bodies in Japan, Europe, USA, and othercountries have jointly organized a project called the 3^(rd) GenerationPartnership Project (3GPP) to produce common radio interfacespecifications for W-CDMA.

The standardized European version of IMT-2000 is commonly called UMTS(Universal Mobile Telecommunication System). The first release of thespecification of UMTS has been published in 1999 (Release 99). In themean time several improvements to the standard have been standardized bythe 3GPP in Release 4 and Release 5 and discussion on furtherimprovements is ongoing under the scope of Release 6.

The dedicated channel (DCH) for downlink and uplink and the downlinkshared channel (DSCH) have been defined in Release 99 and Release 4. Inthe following years, the developers recognized that for providingmultimedia services—or data services in general—high speed asymmetricaccess had to be implemented. In Release 5 the high-speed downlinkpacket access (HSDPA) was introduced. The new high-speed downlink sharedchannel (HS-DSCH) provides downlink high-speed access to the user fromthe UMTS Radio Access Network (RAN) to the communication terminals,called user equipments in the UMTS specifications.

HSDPA is based on techniques such as fast packet scheduling, adaptivemodulation and hybrid ARQ (HARQ) to achieve high throughput, reducedelay and achieve high peak data rates.

Hybrid Arq Schemes

The most common technique for error detection of non-real time servicesis based on Automatic Repeat reQuest (ARQ) schemes, which are combinedwith Forward Error Correction (FEC), called Hybrid ARQ. If CyclicRedundancy Check (CRC) detects an error, the receiver requests thetransmitter to send additional bits or a new data packet. From differentexisting schemes the stop-and-wait (SAW) and selective-repeat (SR)continuous ARQ are most often used in mobile communication.

A data unit will be encoded before transmission. Depending on the bitsthat are retransmitted three different types of ARQ may be defined.

In HARQ Type I the erroneous data packets received, also called PDUs(Packet Data Unit) are discarded and new copy of that PDU isretransmitted and decoded separately. There is no combining of earlierand later versions of that PDU. Using HARQ Type II the erroneous PDUthat needs to be retransmitted is not discarded, but is combined withsome incremental redundancy bits provided by the transmitter forsubsequent decoding. Retransmitted PDU sometimes have higher codingrates and are combined at the receiver with the stored values. Thatmeans that only little redundancy is added in each retransmission.

Finally, HARQ Type III is almost the same packet retransmission schemeas Type II and only differs in that every retransmitted PDU isself-decodable. This implies that the PDU is decodable without thecombination with previous PDUs. In case some PDUs are so heavily damagedthat almost no information is reusable self decodable packets can beadvantageously used.

UMTS Architecture

The high level R9914/5 architecture of Universal MobileTelecommunication System (UMTS) is shown in FIG. 1 (see 3GPP TR 25.401:“UTRAN Overall Description”, available from http://www.3gpp.org). Thenetwork elements are functionally grouped into the Core Network (CN)101, the UMTS Terrestrial Radio Access Network (UTRAN) 102 and the UserEquipment (UE) 103. The UTRAN 102 is responsible for handling allradio-related functionality, while the CN 101 is responsible for routingcalls and data connections to external networks. The interconnections ofthese network elements are defined by open interfaces (lu, Uu). Itshould be noted that UMTS system is modular and it is therefore possibleto have several network elements of the same type.

FIG. 2 illustrates the current architecture of UTRAN. A number of RadioNetwork Controllers (RNCs) 201, 202 are connected to the CN 101. EachRNC 201, 202 controls one or several base stations (Node Bs) 203, 204,205, 206, which in turn communicate with the UEs. An RNC controllingseveral base stations is called Controlling RNC (C-RNC) for these basestations. A set of controlled base stations accompanied by their C-RNCis referred to as Radio Network Subsystem (RNS) 207, 208. For eachconnection between User Equipment and the UTRAN, one RNS is the ServingRNS (S-RNS). It maintains the so-called lu connection with the CoreNetwork (CN) 101. When required, the Drift RNS 302 (D-RNS) 302 supportsthe Serving RNS (S-RNS) 301 by providing radio resources as shown inFIG. 3. Respective RNCs are called Serving RNC (S-RNC) and Drift RNC(D-RNC). It is also possible and often the case that C-RNC and D-RNC areidentical and therefore abbreviations S-RNC or RNC are used.

Evolved Utran Architecture

Currently, the feasibility study for UTRAN Architecture Evolution fromthe current R991415 UMTS architecture is ongoing (see 3GGP TSG RAN WG3:“Feasibility Study on the Evolution of the UTRAN Architecture”,available at http://www.3gpp.org). Two general proposals for the evolvedarchitecture (see 3GGP TSG RAN WG3, meeting #36, “Proposed Architectureon UTRAN Evolution”, Tdoc R3-030678 and “Further Clarifications on thePresented Evolved Architecture”, Tdoc R3-030688, available athttp://www.3gpp.org) have emerged. The proposal entitled “FurtherClarifications on the Presented Evolved Architecture” will be discussedin the following in reference to FIG. 4.

The RNG (Radio Network Gateway) 401 is used for interworking with theconventional RAN, and to act as a mobility anchor point meaning thatonce an RNG 401 has been selected for the connection, it is retained forthe duration of the call. This includes functions both in control planeand user plane.

On the control plane the RNG 401 acts as a signaling gateway between theevolved RAN and the CN, and the evolved RAN and R991415 UTRAN. It hasthe following main functions:

lu signaling gateway, i.e. anchor point for the RANAP (Radio AccessNetwork Application Part) connection,

-   -   RANAP connection termination, including:        -   Setup and release of the signaling connections        -   Discrimination of connectionless messages        -   Processing of RANAP connectionless messages,    -   Relay of idle and connected mode paging message to the relevant        NodeB+(s),

The RNG takes the CN role in inter NodeB+relocations,

User plane control and

lur signaling gateway between NodeB+402-405 and R9914/5 RNC.

Further, the RNG is the user plane access point from the CN orconventional RAN to the evolved RAN. It has the following user planefunctions:

User plane traffic switching during relocation,

Relaying GTP (GPRS tunneling protocol on the lu interface) packetsbetween NodeB+and SGSN (Serving GPRS Support Node, an element of the CN)and

lur interworking for user plane.

The NodeB+402-405 element terminates all the RAN radio protocols (Layer1—Physical Layer, Layer 2—Medium Access Control and Radio Link Controlsub-layers, and Layer 3—Radio Resource Control). NodeB+402-405 controlplane functions include all the functions related to the control of theconnected mode terminals within the evolved RAN. Main functions are:

Control of the UE,

RANAP connection termination,

Processing of RANAP connection oriented protocol messages

Control/termination of the RRC (Radio Resource Control) connection and

Control of the initialization of the relevant user plane connections.

The UE context is removed from the (serving) NodeB+when the RRCconnection is terminated, or when the functionality is relocated toanother NodeB+(serving NodeB+relocation). Control plane functionsinclude also all the functions for the control and configuration of theresources of the cells of the NodeB+402-405, and the allocation of thededicated resources upon request from the control plane part of theserving NodeB+. The “+” in the term “NodeB+” expresses the enhancedfunctionality of the base station in comparison to the R99/415specifications.

User plane functions of the NodeB+402-405 include the protocol functionsof PDCP (Packet Data Convergence Protocol), RLC (Radio Link Control) andMAC (Media Access Control) and Macro Diversity Combining.

Enhanced Uplink Dedicated Channel (E-DCH)

Uplink enhancements for Dedicated Transport Channels (DTCH) arecurrently studied by the 3GPP Technical Specification Group RAN (see3GPP TR 25.896: “Feasibility Study for Enhanced Uplink for UTRA FDD(Release 6)”, available at http://www.3gpp.org). Since the use ofIP-based services become more important, there is an increasing demandto improve the coverage and throughput of the RAN as well as to reducethe delay of the uplink dedicated transport channels. Streaming,interactive and background services could benefit from this enhanceduplink.

One enhancement is the usage of adaptive modulation and coding schemes(AMC) in connection with Node B controlled scheduling, thus anenhancement of the Uu interface. As mentioned in the previous section,in the existing R99/R4/R5 system the uplink maximum data rate controlresides in the RNC. By relocating the scheduler in the Node B thelatency introduced due to signaling on the interface between RNC andNode B can be reduced and thus the scheduler is able to respond fasterto temporal changes in the uplink load. This will reduce the overalllatency in communications of the UE with the RAN. Therefore Node Bcontrolled scheduling is capable of better controlling the uplinkinterference and smoothing the noise rise variance by allocating higherdata rates quickly when the uplink load decreases and respectively byrestricting the uplink data rates when the uplink load increases. Thecoverage and cell throughput may be improved by a better control of theuplink interference.

Another technique, which may be considered to reduce the delay on theuplink, is introducing a shorter TTI (Transmission Time Interval) lengthfor the E-DCH compared to other transport channels. A TTI length of 2 msis currently investigated for use on the E-DCH, while a TTI of 5 ms iscommonly used on the other channels. Hybrid ARQ, which was one of thekey technologies in HSDPA, is also considered for the enhanced uplinkdedicated channel. The hybrid ARQ protocol between Node B and UE allowsfor rapid retransmissions of erroneously received data units, thusreducing the number of RLC (Radio Link Control) retransmissions and theassociated delays. This can improve the quality of service experiencedby the end user.

To support enhancements described above, a new MAC sub-layer isintroduced which will be called MAC-eu in the following. The entities ofthis new sub-layer, which will be described in more detail in thefollowing sections, may be located in UE and Node B. On UE side, theMAC-eu performs the new task of multiplexing upper layer data (e.g.MAC-d) data into the new enhanced transport channels and operating HARQprotocol transmitting entities.

E-DCH MAC Architecture at the UE

FIG. 5 shows the exemplary overall E-DCH MAC architecture on UE side. Anew MAC functional entity, the MAC-eu 503, is added to the MACarchitecture of Rel/99/4/5. The MAC-eu 503 entity is depicted in moredetail in FIG. 6 (see 3GPP TSG RAN WG 1, meeting #31: “HARQ Structure”,Tdoc R1-030247, available of http://www.3gpp.org). There are M differentdata flows (MAC-d) carrying data packets to be transmitted from UE toNode B. These data flows can have different QoS (Quality of Service),e.g. delay and error requirements, and may require differentconfiguration of HARQ instances. Therefore the data packets can bestored in different Priority Queues. The set of HARQ transmitting andreceiving entities, located in UE and Node B respectively will bereferred to as HARQ process. The scheduler will consider QoS parametersin allocating HARQ processes to different priority queues. MAC-eu entityreceives scheduling information from Node B (network side) via Layer 1signaling.

E-DCH MAC Architecture at the UTRAN

In soft handover operation the MAC-eu entities in the E-DCH MACArchitecture at the UTRAN side may be distributed across Node B(MAC-eub) and S-RNC (MAC-eur). The scheduler in Node B chooses theactive users and performs rate control by determining and signaling acommanded rate, suggested rate or TFC (Transport Format Combination)threshold that limits the active user (UE) to a subset of the TCFS(Transport Format Combination Set) allowed for transmission.

Every MAC-eu entity corresponds to a user (UE). In FIG. 7 the Node BMAC-eu architecture is depicted in more detail. It can be noted thateach HARQ Receiver entity is assigned certain amount or area of the softbuffer memory for combining the bits of the packets from outstandingretransmissions. Once a packet is received successfully, it is forwardedto the reordering buffer providing the in-sequence delivery to upperlayer. According to the depicted implementation, the reordering bufferresides in S-RNC during soft handover. FIG. 8 the S-RNC MAC-euarchitecture which comprises the reordering buffer of the correspondinguser (UE) is shown. The number of reordering buffers is equal to thenumber of data flows in the corresponding MAC-eu entity on UE side. Dataand control information is sent from all Node Bs within Active Set toS-RNC during soft handover.

It should be noted that the required soft buffer size depends on theused HARQ scheme, e.g. an HARQ scheme using incremental redundancy (IR)requires more soft buffer than one with chase combining (CC).

E-DCH Signaling

E-DCH associated control signaling required for the operation of aparticular scheme consists of uplink and downlink signaling. Thesignaling depends on uplink enhancements being considered.

In order to enable Node B controlled scheduling (e.g. Node B controlledtime and rate scheduling), UE has to send some request message on theuplink for transmitting data to the Node B. The request message maycontain status information of a UE e.g. buffer status, power status,channel quality estimate. Based on this information Node B can estimatethe noise rise and schedule the UE. With a grant message sent in thedownlink from Node B to the UE, Node B assigns the UE the TFCS withmaximum data rate and the time intervals, the UE is allowed to send.

In the uplink UE has to signal Node B with a rate indicator messageinformation that is necessary to decode the transmitted packetscorrectly, e.g. transport block size (TBS), modulation and coding scheme(MCS) level, etc. Furthermore, in case HARQ is used, the UE has tosignal HARQ related control information (e.g. Hybrid ARQ process number,HARQ sequence number referred to as New Data Indicator (NDI) for UMTSRel.5, Redundancy version (RV), Rate matching parameters etc.)

After reception and decoding of transmitted packets on enhanced uplinkdedicated channel (E-DCH) the Node B has to inform the UE iftransmission was successful by respectively sending ACK/NACK in thedownlink.

Mobility Management within R9914/5 UTRAN

In this section some frequently used terms will be briefly defined andsome procedures connected to mobility management will be outlined (see3GPP TR 21.905: “Vocabulary for 3GPP Specifications” available athttp://www.3gpp.org).

A radio link may be a logical association between single UE and a singleUTRAN access point. Its physical realization comprises radio bearertransmissions.

A handover may be defined as transfer a users connection from one radiobearer to another. In a “hard handover” of a new radio link isestablished. In contrast, during “soft handover” (SHO) radio links areestablished and abandoned such that the UE always keeps at least oneradio link to the UTRAN. Soft handover is specific for networksemploying Code Division Multiple Access (CDMA) technology. Handoverexecution is commonly controlled by S-RNC in mobile radio network.

The “active set” comprises a set of radio links simultaneously involvedin a specific communication service between UE and radio network, e.g.during soft handover, the UE's active set comprises all radio links tothe RAN's Node Bs serving the UE. Active set update procedures may beused to modify the active set of the communication between UE and UTRAN.The procedure may comprise three functions: radio link addition, radiolink removal and combined radio link addition and removal. The maximumnumber of simultaneous radio links is commonly set to four. New radiolinks may be added to the active set once the pilot signal strengths ofrespective base stations exceed certain threshold relative to the pilotsignal of the strongest member within active set. A radio link may beremoved from the active set once the pilot signal strength of therespective base station exceeds certain threshold relative to thestrongest member of the active set.

The threshold for radio link addition may be typically chosen to behigher than that for the radio link deletion. Hence, addition andremoval events form a hysteresis with respect to pilot signal strengths.

Pilot signal measurements are reported to the network (S-RNC) from UE bymeans of RRC signaling. Before sending measurement results, somefiltering is usually performed to average out the fast fading. Typicalfiltering duration is about 200 ms and it contributes to handover delay(see 3GPP TS 25.133: “Requirements for Support of Radio ResourceManagement (FDD)”, available at http://www.3gpp.org). Based onmeasurement results, S-RNC may decide to trigger the execution of one ofthe functions of active set update procedure (addition/removal of a NodeB to/from current Active Set).

E-DCH—Operation During Soft Handover

Supporting soft handover is desirable to obtain the macro diversitygain. In HSDPA for example no soft handover is supported for the HS-DSCH(High Speed Downlink Shared Channel) transport channel. Applying softhandover causes the problem of distributing scheduling responsibilitiesacross all Node Bs of the active set and would require extremely tighttiming to provide the scheduling decision to all members of the activeset even if distribution of scheduling function were resolved. Only oneNode B is transmitting on HS-DSCH to a UE and thus no macro diversitygain is exploited. When UE enters soft handover region for dedicatedchannels, the Node B, which is allowed to transmit on HS-DSCH, has to bedetermined. The selection of serving Node B may be done from either theUE side or from network side (by RNC).

In the Fast Cell Selection (FCS) method for HS-DSCH, the UE selects thecell that is the most suitable for transmitting data. UE periodicallymonitors the channel conditions in the cells within the active set tocheck whether there is a cell with better channel conditions than thecurrent serving cell.

In case soft handover is not supported for the uplink, a serving Node Bhas to be selected. One problem, which might occur, is inaccurateselection of the serving Node B. Thus there may be a cell within activeset more suitable for uplink transmission than the chosen uplink servingNode B. Therefore, data transmission to a cell controlled by currentserving Node B could fail, whereas the transmission to the cellscontrolled by other Node Bs would have been successful. The accuracy ofthe selection depends on several factors like signaling delay, filteringof measurement results etc.

To conclude, supporting SHO operation for E-DCH is useful because ofmacro diversity gain and because possible transmission failures due toan inaccurate selection of the best uplink serving Node B can beeliminated.

Soft Handover Operation without Soft Buffer Synchronization

A flow chart for Node B soft handover operation without soft buffersynchronization assuming R99/R4/R5 architecture is given in FIG. 9. Thefigure depicts the operation of an arbitrary Node B from the Active Set.

Each Node B within active set monitors the enhanced dedicated physicaldata channel (E-DPDCH) in step 901 for the reception of uplink traffic.In case a packet is received in step 903 within a transmission timeinterval (TTI) (see step 902), Node B has to decide if the packet wasthe initial transmission or a retransmission of a previously sent datapacket. The decision is based on associated uplink control signaling,e.g. the New Data Indicator (NDI). In case the received packet was aretransmission then Node B has to combine the received data packet withthe previous transmissions stored in the soft buffer before decoding instep 905. For an initial transmission Node B stores (see step 906) thereceived packet in the corresponding soft buffer (possible previoustransmissions stored in the that soft buffer are overwritten) and canimmediately try to decode the packet upon reception.

The testing whether decoding was successful or not (see step 907) isdone by evaluating the CRC checksum. If the packet is correctly decoded,Node B passes it to higher layer and sends it to S-RNC via lub/lurinterface in step 908. In case decoding was not successful the softinformation is stored in the soft buffer in step 909.

As outlined above, soft handover operation provides an additional macrodiversity gain but also complicates system design to a certain extent.Taking the E-DCH as an example, there is a single transmitting protocolentity and multiple receiving protocol entities for soft handoveroperation, while for non-soft handover operation there are only a singletransmitting and a single receiving protocol entity.

Radio Bearer Establishment

Before starting of any transmission the radio bearer may be establishedand all layer should be configured accordingly (see 3GPP TS25.331 RadioResource Control (RRC) protocol specification”, available athttp//www.3gpp.org). The procedures for establishing radio bearers mayvary according to the relation between the radio bearer and a dedicatedtransport channel. Depending on the QoS (Quality of Service) parameters,there may or may not be a permanently allocated dedicated channelassociated with the RB.

Radio Bearer Establishment with Dedicated Physical Channel Activation

In UMTS the procedure in FIG. 10 may be used when a new physical channelneeds to be created for the radio bearer. A Radio Bearer Establishmentmay be initiated when an RB Establish Request primitive is received fromthe higher layer Service Access Point on the network side of the RRClayer. This primitive may comprise a bearer reference and QoSparameters. Based on these QoS parameters, Layer 1 and Layer 2parameters may be chosen by the RRC entity on the network side.

The physical layer processing on the network side my be started with theCPHY-RL-Setup request primitive issued to all applicable Node Bs. If anyof the intended recipients is/are unable to provide the service, it maybe indicated in the confirmation primitive(s). After setting up Layer 1including the start of transmission/reception in Node B, the NW-RRC maysend a RADIO BEARER SETUP message to its peer entity (acknowledged orunacknowledged transmission optional for the NW). This message maycomprise Layer 1, MAC and RLC parameters. After receiving the message,the UE-RRC configures Layer 1 and MAC.

When Layer 1 synchronization is indicated, the UE may send a RADIOBEARER SETUP COMPLETE message in acknowledged-mode back to the network.The NW-RRC may configure MAC and RLC on the network side.

After receiving the confirmation for the RADIO BEARER SETUP COMPLETE,the UE-RRC may create a new RLC entity associated with the new radiobearer. The applicable method of RLC establishment may depend on RLCtransfer mode. The RLC connection can be either implicitly established,or explicit signaling can be applied. Finally, an RB EstablishIndication primitive may be sent by UE-RRC and an RB EstablishConfirmation primitive may be issued by the RNC-RRC.

A simple HARQ operation is currently only possible for a communicationbetween a single transmitter and a single receiver in case of ensuringreliable feedback transmission. The feedback transmission ensures thatsender and receiver are synchronized. By increasing the sequence numbervalue of a window based HARQ process or toggling the New Data Indicator(NDI) of a stop-and-wait (SAW) HARQ process in the associated HARQcontrol information the receiver knows if a new packet is being receivedand if it can flush the soft buffer accordingly.

This ensures that a new packet will not be combined with a previouslystored packet in the receiver. A wrong combining of packets beforedecoding may be a rare case, but cannot be completely excluded iffeedback signaling is not entirely reliable. A correct decoding will notbe possible in that case.

Hence the receiver may request for a retransmission of that packet bysignaling a NAK. Retransmission of this packet may go on until themaximum number of retransmissions is reached. If there are manyretransmissions of a ‘new’ packet which was combined with previous softbuffer values of an ‘old’ packet the influence of the soft values of the‘old’ packet may be reduced due to successive combining with the newpacket allowing a successful decoding of the new packet. How strong thethroughput is affected by packet retransmissions may depend on thelikelihood of an erroneous operation of the packet retransmissionprocedure. There may be a trade-off between the overhead spent forreliable signaling and likelihood for erroneous protocol operation. Inthe same way there may be a procedure to inform the receiver whether apacket has been aborted by the transmitter. This could for instance becaused by reaching the maximum number of retransmissions or in case theassigned delay attribute (or Ume to live value) could not be met.

Some communication systems as Wideband Code Division Multiple Access(W-CDMA) rely on soft handover operation. In addition to the problemthat now multiple feedbacks of each receiver need to be receivedcorrectly there is also the problem to synchronize the HARQ soft bufferbetween the transmitter and a multiplicity of receivers. Not all Node Bsmay be able to receive the associated control signaling from the UE,which is needed for a correct processing of the received packet.Assuming that the control information has been received Node B can tryto decode the packet and buffer the soft values in case a successfuldecoding is not possible. It is likely that there is one Node B (e.g.the best link) that is able to decode the packet whereas others do notreceive anything.

Transmission of new packets will continue to the best Node B while thereare still old packets buffered at other receivers.

In WO 92/37872 a method is introduced that unveils the HARQ operationduring soft handover from one transmitter to multiple receivers in theuplink. Reception cannot be guaranteed since power control and thustransmit power is usually adapted to the best link within Active Set.That means as well that reliable feedback from all the receivers isdifficult to achieve The transmit power in the uplink needs to beincreased for the “bad” links to ensure a well synchronized operationwhich will increase the uplink interference significantly. WO 92/37872proposes to increase the HARQ protocol reliability by adding a flush bitto the associated HARQ uplink control information.

A set flush bit informs the receiver not to combine the packet withprevious transmissions, but to flush the HARQ soft buffer of that HARQprocess. This works in principle, but has two drawbacks. Firstly itassumes that the transmitter knows the state of the receiver, because ithas to inform it when to flush the buffer. If the transmitter is notsure about the receiver state due to unreliable or missing feedback thebuffer should be flushed. This will lead to loss of information in casethe packet had already been received and stored in the soft buffer.Secondly it needs to transmit that flush bit with high reliability alongthe HARQ control information. This will increase the over the airsignaling overhead in the uplink.

The problems of non-synchronized buffers during a soft handoveroperation with multiple base stations operating as receivers has beendescribed in detail. Existing solutions rely, besides on regular HARQcontrol information such as HARQ process and HARQ sequence number orNDI, on additional signaling to flush the soft buffer and avoiderroneous combining.

SUMMARY OF THE INVENTION

The object of the present invention is to prevent erroneous combining ofdata packets in a packet retransmission scheme at the receiver. Theerroneous combining may be caused by non-synchronized soft buffers ofmultiple receivers.

The object of the present invention is solved by the subject matter ofthe independent claims. Preferred embodiments of the present inventionare defined in the dependent claims.

Taking a window based HARQ protocol as an example for a data packetretransmission scheme, it should not happen that a packet is receivedwith same sequence number as an old packet in the soft buffer. Thisphenomenon is called wrap around problem. The HARQ window is advancedwhile the soft buffer of that sequence number is not flushed. ForN-channel Stop-and-Wait protocol the issue is the similar. The same HARQprocess should not be scheduled again with a new packet unless this isindicated and the soft buffer is flushed.

The present invention may ensure a correct protocol operation withmultiple base stations as data receivers while avoiding additionalsignaling over the air interface or within the network. In a first stepeach buffer may be flushed after each successful decoding of a receiveddata packet or a combination of an erroneous data packet andretransmissions relating thereto. In addition or alternatively to theimmediate buffer flush upon correct reception of a data packet, the timeelapsed since the last storing in a particular buffer region may bemonitored in each base station, e.g. by means of a timer or counter. Themonitoring may ensures that old packets in the soft buffer are flushedbefore a new packet is received.

A threshold time period, i.e. the maximum allowable time period afterwhich no retransmission of a data packet may arrive at a base stationmay be predetermined or configured. After the expiry of this timeperiod, an associated buffer region in the base station is flushed andnew data packet may be received. Configuration of the threshold timeperiod may be done by higher layer signaling between a communicationterminal, such as an UE, and a receiver, such as a base station. Thestarting value of the timer may correspond to the threshold time period.

Hence, the communication terminal may “know” about the time when abuffer region for a particular data packet and its relatedretransmissions, will be flushed at a base station it communicates with.Therefore, it may know until what point in time a retransmission of aspecific data packet or retransmission data packet has to be received atthe base station to gain from soft combining. If the buffer has beenflushed in the receiver the communication terminal may use thatknowledge in selecting the correct transmission parameter for a newtransmission of the aborted data packet.

The present invention provides a method for use in a packetretransmission scheme in a mobile communication system comprising acommunication terminal and a plurality of base stations, wherein thecommunication terminal is in communication with the plurality of basestations during a soft handover. The method may comprise the steps ofreceiving a data packet from the communication terminal at the pluralityof base stations and checking data integrity of the received data packetat each of the base stations. If data integrity of the received datapacket was not confirmed by a base station, the received data packet maybe stored in a region of a buffer of the respective base station,wherein the buffer region is associated with said received data packet.The time elapsed since the storing of the data packet in the associatedbuffer region may be monitored. It should be noted that the plurality ofbase stations may not refer to all base stations that are controlled bya control unit or a plurality of control units in the mobilecommunication network, but rather to the base stations communicatingwith the communication terminal during soft handover. In UMTS thisplurality of base stations may be referred to as the active set of thecommunication terminal. Hence, the plurality of base stations may be asubset of the base stations available for communication in the mobilecommunication network.

If data integrity of said received data packet was confirmed, theassociated buffer region my be flushed in respective base station.

As an alternative solution of the object as stated above, the presentinvention further provides a method for updating the soft buffer of abase station in a mobile communication system comprising a communicationterminal and a plurality of base stations. According to this embodiment,the communication terminal is in communication with the plurality ofbase stations during a soft handover. According to the method, a datapacket from the communication terminal may be received at the pluralityof base stations.

Further, data integrity of said received data packet may be checked ateach of the base stations, and if data integrity of said received datapacket was confirmed, a buffer region associated with said received datapacket may be flushed.

In another embodiment of the present invention, if data integrity ofsaid received data packet was not confirmed by a base station, thereceived data packet may be stored in an associated region of a bufferof the respective base station, and the time elapsed since the storingof said data packet in said associated buffer region ma be monitored.

If the respective monitored time period is equal to or larger than athreshold time period after which a retransmission data packet can nolonger be expected in the respective base station the buffer region maybe flushed. The data packet may for example be received via a dedicatedchannel.

In case the data integrity of an initial transmission of a data packetor the integrity of a retransmission data packet was not confirmed by abase station a retransmission data packet may be requested in accordancewith a packet retransmission scheme. Hence, in a further embodiment, aretransmission data packet may be received from the communicationterminal at the plurality of base station. Upon reception, a basestation may perform a data integrity check of the receivedretransmission data packet at each of the base stations, and if dataintegrity was not confirmed by the base station, the retransmission datapacket may be stored in the buffer region associated with a previousdata packet relating to the retransmission data packet, and themonitoring of the time elapsed since the storing of the retransmissiondata packet in the associated buffer region may be restarted.

The data integrity check performed on the retransmission data packet maycomprise combining the retransmission data packet with the related datapacket to obtain a combined data packet, decoding the combined datapacket to obtain decoded data, and checking the integrity of the decodeddata. In more general terms, checking the data integrity may be done byverifying the incorrupbon of the received data corresponding to a(re)transmission process of a specific data packet, e.g. by means of acyclic redundancy check (CRC).

If data integrity of a received data packet was confirmed, theassociated buffer region may be flushed.

Upon receiving a retransmission data packet from the communicationterminal at the plurality of base stations, a data integrity check ofthe received retransmission data packet at each of the base stations maybe performed, and if data integrity of was confirmed by a base station,the monitoring of the time elapsed since the storing of the transmissiondata packet in the associated buffer region may be stopped. It isfurther, noted, that in the case above the retransmission data packetmay be stored in the buffer region. The term data packet may beunderstood as a generic expression referring to a retransmission packetor an initial transmission.

If the respective monitored time period is equal to or larger than athreshold time period, the monitoring of the respective data packet maybe also stopped, as it is not very likely to receive a retransmissiondata packet for the data packet associated to the buffer region. Byflushing the buffer region it may be ensured that when reusing thebuffer region a new data packet is not combined with the “old”content—i.e. a data packet and its related retransmissions receivedpreviously—of this buffer region.

As outlined above it is desirable that the threshold time period is ofconfigurable duration. Signaling the duration of the threshold timeperiod to at least one of the plurality of base stations may beaccomplished by using radio network control signaling from a controlunit in the mobile communication network. For example, when employing aRAN according to the UMTS specifications, the duration of the thresholdtime period may be signaled to the at least one base station in aninformation element of a NBAP (Node B Application Part) message.

Further, the signaling of the duration of the threshold time period tothe communication terminal may be accomplished by radio resource controlsignaling from a control unit in the mobile communication network.Again, when employing a RAN according to the UMTS specifications, theduration of the threshold time period may signaled to the communicationterminal in an IE (information element) of at least one of a radiobearer setup message, radio bearer reconfiguration message, radioresource control connection setup message, transport channelreconfiguration message, cell update message, and a handover commandmessage.

In accordance with a packet retransmission scheme, e.g. HARQ, thereception status of a data packet may be indicated to the communicationterminal. Therefore, a message from at least one of the plurality ofbase stations may be transmitted to the communication terminalindicating whether at least one of the plurality of base stationsconfirmed data integrity of the received data packet.

A successfully received and decoded data packet may be forwarded to ahigher layer for further processing. Hence, according to an embodimentof the present invention the received data packet is transmitted to acontrol unit of the mobile communication system by at least one of thebase stations that did confirm data integrity of the received datapacket.

As the communication terminal may not have sufficient capacity assignedfor retransmitting a corrupted data packet before a buffer region flushat a base station, it may signal to the base station to increase itsassigned capacity for the retransmission data packet. A base stationtherefore receives a capacity request message from the communicationterminal requesting additional transmission capacity for aretransmission of a data packet.

Advantageously, the capacity request message comprises at least one of atransmission priority of a data packet to be transmitted by thecommunication terminal, the size of data in a transmission buffer of thecommunication terminal and the duration of the monitored time period.These parameters may advantageously be used by the base station todecide whether to increase the assigned channel capacity for therequesting communication terminal or not. Alternatively, according to afurther embodiment of the present invention, the capacity request of thecommunication terminal may include HARQ side information, e.g. sequencenumber, HARQ process or New Data Indicator to identify the packet forwhich capacity is requested. For the packet indication the base stationmay know some of the corresponding parameter of that packet such as forexample the threshold time period and the priority of the data packet.Similarly the communication terminal may identify the physical channel,the transport channel and/or the logical channel for which it isrequesting capacity.

In response to the capacity request message or in case the base stationis able to increase the capacity assigned to the communication terminal,a capacity grant message is transmitted to the communication terminal,wherein the capacity grant message indicates a transmission capacityassigned to the communication terminal for data transmission.

Another possibility to prevent the flushing of a buffer regionassociated to a specific data packet and its related retransmission datapackets, may be to transmit a restart request message to a base station,wherein the restart request message indicates a data packet for whichmonitoring of the time elapsed since the storing of the data packet (ora related retransmission data packet) in the associated buffer regionhas to be restarted. A base station may receive this restart requestmessage and restart the monitoring. The restart request message maycomprise control information and no or dummy payload data. In anotheralternative embodiment of the present invention, upon expiry of thethreshold time period, a base station may mark an associated bufferregion of the packets as a buffer region to be flushed. If a new packetassociated to that buffer region (e.g. identfied by the sequence number)is received it may finally flush the soft buffer unless it received someadditional control information. Such control information may be acombine indicator. A combine indicator may be realized as flag which maybe sent if the transmitted data packet should be combined. In the lattercase the marked buffer region may not be flushed and a combining willstill take place although the timer already expired. This may allow fora soft combing even if a retransmission of a data packet is delayed.

When using a window based packet retransmission scheme, the method mayfurther comprise the step of calculating the threshold time period basedon the time required for the transmission of all data packets within awindow of the packet retransmission scheme.

Independent of the retransmission scheme used, the threshold time periodmay also be calculated based on the time interval between the receptionof an initial data packet and the reception of a retransmission datapacket.

According to another embodiment, the calculation of the duration of thethreshold time period may be based on at least one of the followingparameters: size of the buffer, the maximum number of packetretransmissions in a data packet retransmission scheme, thecommunication terminal's processing time for a feedback message, therespective base station's processing time for a received data packet anda transmission time interval.

The present invention further provides a base station in a mobilecommunication system comprising a communication terminal and a pluralityof base stations, wherein the communication terminal is in communicationwith the plurality of base stations during a soft handover, and whereinthe base station comprises means for implementing the method describedabove.

In another embodiment, the present invention provides to a method forscheduling data retransmissions in a communication terminal being partof a mobile communication system comprising the communication terminaland a plurality of base stations, wherein the communication terminal isin communication with the plurality of base stations during a softhandover. The method may comprise the steps of transmitting a datapacket to the plurality of base stations, receiving at least onefeedback message from at least one of the base stations, evaluating theat least one feedback message to determine whether data integrity of thetransmitted data packet was confirmed by at least one of the pluralityof base stations, and if data integrity of the transmitted data packetwas not confirmed by a base station, monitoring the time period elapsedsince the transmission of the data packet or the reception of therespective feedback message, to schedule a retransmission relating tothe data packet to occur not after expiry of a threshold time periodafter which a reception of a retransmission data packet can no longer beexpected in the respective base station.

If data integrity of the transmitted data packet was not confirmed andat a point in time where the monitored time period is smaller than thethreshold time period, a capacity request message may be transmitted tothe plurality of base stations requesting further transmission capacityfor a retransmission of a data packet as already outlined above.

If a capacity grant message indicating a transmission capacity assignedto the communication terminal for data transmissions was not receivedfrom a base station of the plurality of base stations or no additionalcapacity was assigned to the UE in response to a capacity requestmessage, a restart request message may be transmitted from thecommunication terminal to a base stations, wherein said restart requestmessage indicates a data packet for which monitoring at the respectivebase station has to be restarted.

The usage of a restart request message is not bound to the results of arequest for additional transmission capacity. According to anotherembodiment of the present invention, if data integrity of thetransmitted data packet was not confirmed and at a point in time wherethe monitored time period is smaller than the threshold time period, arestart request message may be transmitted to a base stations, whereinthe restart request message indicates a data packet for which monitoringat the respective base station has to be restarted.

As will be explained further down below in more detail, in case aretransmission data packet may not be transmitted from the communicationterminal before the associated buffer region at the base station will beflushed, e.g. due to insufficient capacity assigned, the communicationterminal may await the flushing of the buffer region to transmit a newinitial data packet. Therefore, the method may comprise the step ofstalling the retransmissions a data packet until the respectivemonitored time period is larger than the threshold time interval, ifdata integrity of the transmitted data packet was not confirmed and at apoint in time where the monitored time period is smaller than thethreshold time period.

In order to initiate the transmission of a retransmission data packet ifdata integrity of the transmitted data packet was not confirmed, ascheduler in the communication terminal is informed and may reschedulethe transmitted data packet for retransmission. In analogy, if dataintegrity for the transmitted data packet was confirmed, the schedulermay be informed by the communication terminal in order to remove thetransmitted data packet from a transmission buffer of the communicationterminal.

When a retransmission relating to the initial data packet becomesnecessary, the communication terminal may transmit the retransmissiondata packet to the plurality of base stations, and in turn receives atleast one feedback message from at least one of the base stations. Next,the at least one feedback message may be evaluated to determine whetherdata integrity of the transmitted retransmission data packet wasconfirmed by at least one of the plurality of base stations, and if dataintegrity was confirmed, the monitoring of the time elapsed since thetransmission of the a data packet or the reception of a respectivefeedback message may be restarted. E.g. when using a timer formonitoring the timer is reset to its initial threshold value and isstarted again. Further the present invention provides a communicationterminal in a mobile communication system comprising a communicationterminal and a plurality of base stations, wherein the communicationterminal is in communication with the plurality of base stations duringa soft handover, and wherein the communication terminal comprises meansfor implementing the method steps as outlined before.

According to another embodiment of the present invention, thecommunication terminal and the base station as described above may beadvantageously be combined in a mobile communication system.

BRIEF DESCRIPTION OF THE FIGURES

In the following the present invention is described in more detail inreference to the attached figures and drawings. Similar or correspondingdetails in the figures are marked with the same reference numerals.

FIG. 1 shows the high-level architecture of UMTS,

FIG. 2 shows the architecture of the UTRAN according to UMTS R99/415,

FIG. 3 shows a Drift and a Serving Radio Subsystem,

FIG. 4 shows the evolved UTRAN architecture.

FIG. 5 shows the E-DCH MAC architecture at a UE,

FIG. 6 shows the MAC-eu architecture at a UE,

FIG. 7 shows the MAC-eu architecture at a Node B,

FIG. 8 shows the MAC-eu architecture at a RNC,

FIG. 9 shows a prior-art flow chart of HARQ receiver operation,

FIG. 10 shows a setup procedure of a radio bearer according to the UMTSspecifications,

FIG. 11 shows a flow chart of the operation of a base station accordingto an embodiment of the present invention,

FIG. 12 shows a flow chart of the operation of a communication terminalaccording to an embodiment of the present invention, and

FIG. 13 illustrates the timing of the transmission of data between acommunication terminal and a base station according to an embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

It is noted that the different embodiments in the following will bedescribed mainly in 25 relation to the HARQ packet retransmission schemeand UMTS. Nevertheless, the principles underlying the present inventionare also applicable to other data packet retransmission schemes and toother mobile communication systems than UMTS that provide soft handoverof communication terminals and packet retransmission mechanisms.

In FIG. 11 shows an illustrative flow chart of the operation of a basestation within the active set of a UE during soft handover. The basestation may monitor the physical channels in step 1101 and may regularlycheck whether one or more data packets are received within atransmission time interval (TTI) in step 1102. If a data packet is sentto the base station, it is received in step 1103 and upon reception thebase station decides, whether the received data packet is an initialdata packet or a retransmission relating to an initial data packet instep 1104. Further, in case the received data packet is a retransmissiondata packet the base station will combine the retransmitted data withrelated soft values stored in an associated soft buffer region of thebase station in step 1105. For example if the initial data packet hasnot been received correctly, i.e. its data is corrupted and could not bedecoded by the base station, the retransmission data relating to thisinitial data packet is combined with the data from this initial datapacket and the combined data packet is decoded in step 1106. In case aninitial data packet is received, the packet can be decoded directlywithout previous combination in step 1106. Step 1106 further checks thedata integrity of the decoded data.

If the data integrity is confirmed, the flow advances to block 1107. Asa first improvement compared to the state of the art packetretransmission scheme as illustrated in FIG. 9, a buffer regionassociated to a data packet and its possible retransmission, e.g. a HARQsoft buffer, will be deleted or flushed immediately after the datapacket is received correctly in step 1107 and not only after the a newpacket is received as in conventional packet retransmission schemes(confer block 906 in FIG. 9). Hence, the buffer region associated to aninitial data packet and its retransmissions is immediately freed uponcorrectly decoding which ensures that no data from previously receiveddata packets reside in the buffer region upon receiving a new datapacket that will be associated with this buffer region.

For data transmissions between a communication terminal and a pluralityof base stations the immediate flushing of a soft buffer region can makea difference since some Node B may temporarily not receive controlinformation and the Node B, which is receiving uncorrupted data packetsfrom the UE, may go on with the transmission for some time.

If the buffer region is e.g. flushed immediately after correct decodingof data packet an erroneous combining may be excluded from thebeginning. Additionally, a timer that will be started preferably for allnew packets (n=0, 1 . . . N) not successfully decoded may used toprevent wrong combining. After or before the soft values of a datapacket, i.e. an initial data packet or a retransmission data packet willbe stored in the soft buffer, a timer is started to indicate how longthe packet has been stored in the buffer or rather how long it can stillbe kept in the soft buffer before the associated buffer region isflushed. During regular operation with continuous reception aretransmission may be received before the timer expires and the timerfor that data packet Xi will be restarted. If a packet is receivedcorrectly, besides flushing the associated buffer region 1107, the timermay be stopped for that data packet.

In case a data packet, i.e. the initial data packet alone or acombination of an initial data packet and soft values from one or moreretransmission related to this packet, can not be decoded correctly instep 1106, the newly arrived data packet is stored in an associated softbuffer region.

Taking HARQ as an example, in each received data packet, no matterwhether an initial data packet or a retransmission data packet, the HARQprocess number and the NDI identifies a retransmission of a particulardata packet as outlined above. If a data packet of with particularprocess number is received and can not be decoded correctly, the datapacket's soft values may be stored in an associated buffer regiontogether with other data from packets containing the same HARQ processnumber.

In case a timer for a data packet that may be stored to an associatedbuffer region is not running, i.e. an initial data packet has beenreceived, the timer associated with the buffer region and the receiveddata packet will be started in step 1111. In case a timer is alreadyrunning for received data packet's associated buffer region the timermay be restarted (see also step 1111).

Before going to the reception of the next transmission time interval(TTI) all timers may be decremented in step 1112. The timers may bedecremented independent of whether a data packet has been received inthe elapsed TTI or not.

If it is judged in step 1113 that any of the timers expired, theparticular buffer region comprising the initial data packet and,possibly, additional soft values from related retransmission packetsreceived, will be flushed in step 1114, as the respective base stationcan no longer expect a retransmission data packet relating to thecorrupted data packet stored in the soft buffer region. If no timerexpired the next data packet is expected for reception.

One criteria for setting the timer value i.e. for setting a thresholdtime period after which a retransmission data packet is no longerexpected in a base station, is to make it small enough to avoid anerroneous combining of different packets. At the same time the timervalue should be large enough to avoid a flush of the stored packets forwhich retransmissions are still pending.

For example in HARQ protocols as used in UMTS Rel. 5, HARQ transmissionsmay be asynchronous, while ACK/NAK feedback messages may be sentsynchronous. This means that it is usually not exactly known to a basestation when a retransmission of an initial data packet will be sent bythe communication terminal, which makes the setting of the timer ratherdifficult. In case the UE may transmit data packets autonomously, it maybe likely that the retransmission data packet will follow shortly afterthe transmission of the initial data packet, taking into account theprocessing time at the base station and the communication terminal.Retransmissions may be sent with higher priority to minimize delay anddelay jitter. Furthermore a retransmission should not be pending for toolong since the channel conditions may change and thus the transmissionformat, e.g. packet size, modulation, coding rate, etc. may not besuitable for the channel conditions anymore. Latter case may require anadaptation of the transmission format (TFC—transmission formatcombination) to the new channel conditions.

The maximum timer value allowable may depend on the detailed packetretransmission protocol design. For example, in a window based HARQprotocol the wrap around problem should be avoided. For reliable ARQoperation the ARQ window may be at least twice the size of thetransmitter or receiver window size. As soon as a data packet outside ofthe window is received the window will be forwarded. Assuming thesequence numbers are used in order to identify a certain position insidethe HARQ window, the window is not forwarded in larger steps and allsubsequent data packets are received correctly, the timer value may becalculated considering how long it takes to transmit all the packets ofthe window. In the latter case, the timer would expire before a newtransmission of a new data packet with the same sequence number would beinitiated.

The larger the window size, or the more HARQ SAW processes or the largerthe window per HARQ process, and thus the soft buffer size, the largerthe timer value may be. For HARQ the soft buffer size may be one of themain complexity factors since each soft value of a retransmission datapacket needs to be buffered in a buffer region. Therefore the windowsize may not be over dimensioned. This means that the buffer may bedimensioned to store as many packets as being received during the roundtrip time in continuous transmission. Depending on the receiverimplementation the packet may be buffered by means of bit level orsymbol level soft values. For higher level modulation (e.g. 16 QAM)buffering in symbol level requires less memory whereas bit level softvalues allow for the highest flexibility if for instance certain bitsneed to be punctured. Another design criteria is the bit granularityi.e. how many bits represent one soft value. Hence, there may be atradeoff between accuracy and buffer size.

The round trip time, illustrated in FIG. 13, may be defined as the timefrom the initial transmission 1301 of a data packet from the UE until aretransmission of the same data packet 1305 upon receiving a negativefeedback message. This involves two times the propagation delayt_(propa) plus UE and Node B processing times, t_(UE) _(—) _(process)and t_(NodeB) _(—) _(process). Assuming that a retransmission can besent 6 TTI's after the previous or initial transmission. To realizecontinuous transmission, this may correspond to a transmitter andreceiver window size of at least 6. In a pure window based ARQ systemthe minimum overall window size may be at least two times thereceiver/transmitter window size, i.e in the example equal to 12. Thismay ensure a unique identification of packets and a correct ARQoperation even if some ACK/NAKs are lost. If there is for instance a NAKto ACK misinterpretation a new packet may not have the same sequencenumber, but the window would be probably moved indicating to thereceiver that this is not the expected retransmission, but a new packet.

The same calculation may be done for N-channel SAW protocols. Thepreviously described ARQ system is equivalent to a 6 channel SAWprotocol and a one bit New Data Indicator (equal to a sequence number).In both system the timer may be set to 12 times the TTI to make theprotocol work for the worst case scenario. Assuming synchronoustransmission as illustrated in the FIG. 13 the maximum timer value forthe worst case may be calculated, which will be explained in thefollowing:

The UE may transmits a first data packet with a process number set to 1and NDI equal to 0 to two Node Bs, whereby Node B1 receives data packet,but can not decode it and thus buffers the soft values in a bufferregion of the soft buffer and sends an NAK to the UE to indicate thefailed decoding. Node B2 may decode the data packet successfully andsends an ACK. Hence, the UE receives an ACK from Node B2 and the NAKfrom Node B1.

Further, it is assumed that the UE sends packets 2, 3, . . . , 12 toNode B1 and Node B2, wherein Node B2 receives and successfully decodesall packets. Data packets 1 to 6 may comprise different HARQ processnumbers and an NDI set to 0. Packets 7 to 12 are transmitted with theNDI set to 1.

Node B1 may be temporarily out of reception and missed packet 7 withprocess number 1 and an NDI set to 1. Hence, the Node B1 still has thefirst data packet with a process number of 1 and an NDI set to 0 storedin the associated soft buffer.

The UE sends new data packet 13 with process number 1 and NDI equal to0, which is received by Node B1. As the process number and NDI of thenew data packet is identical with the process number and NDI of the datapacket stored in the buffer, Node B1 combines the two packets as the newdata packet is considered to be a retransmission. Hence in a situationas outlined above, Node B1 would erroneously combine data packet 13 withthe data of first data packet 1 stored in the soft buffer.

This is a worst case calculation based on very specific assumption whichmay rarely occur in a real system. First of all the UE may not bescheduled continuously as in the above example. The misinterpretation ofthe feedback signaling or to completely miss of a packet should also bean exceptional event. Also the ARQ operation may not be synchronous,i.e. retransmission may not be sent after expiry of a fixed time period.There may be retransmissions of some other packets pending which have ahigher priority as initial transmission and would supersede new initialtransmissions further increasing the time until a wrap around couldhappen. The limited window size will cause the need of the HARQ bufferto be flushed only after one missed reception.

The packet retransmission scheme or protocol used may be configurable ina flexible way considering for instance Quality of Service (QoS)requirements of specific data flows. This may be for instance a certainbit error rate to be reached or a delay requirement. For example, whenusing the HARQ protocol configurable parameters may be the size of theHARQ soft buffer, the number of HARQ processes, the time until a packetis aborted, the maximum number of retransmissions, the minimum inter TTIinterval or the UE and/or Node B processing time etc. The setting of thetimer, when to clear a packet from the soft buffer, may depend on suchkind of parameter which should be considered as well.

The problem of non-synchronized Node B buffers may occur during softhandover when two or more Node Bs are receiving and trying to decode thesame packet. Therefore RNC may inform the Node B about the soft handoverstate of each particular UE. A timer may only be started in case the UEis in soft handover.

Further, the Node B may be given a default timer value or the value maybe configured by higher layer such as Radio Resource Control. The RNCmay signal a message with a new information element e.g. calledHARQ_flush_timer. The message may be transmitted to set up or modify aspecific physical or transport channel e.g. an Enhanced DedicatedChannel. In UMTS the radio network control protocol is called Node BApplication Protocol (NBAP). For enhanced dedicated uplink packettransmission, different scheduling options may be employed. If the NodeB controls uplink transmission of the UEs it may prioritize a certain UEthat has a timer which is about to expire. This may enable the UE tosend the retransmission data packet before timer expiry and a softbuffer flush at the base stations of its active set and to gain fromsoft combining.

Although retransmissions may have a higher priority the UE schedulingdecision may consider other parameters such as channel quality,available transmit power, different priorities of different data flowsetc. If the UE can schedule some transmission autonomously or mayrequest additional capacity for transmission, it may prioritize packetsof which the timer is about to expire.

Therefore in another embodiment, the timer value may be known to the UEe.g. it may be predetermined. In another embodiment of the invention,the timer value is signaled to the UE. The timer value may be signaledusing RRC signaling. This may require the definition of a newinformation element, e.g. called HARQ_flush_timer, for an RRC signalingmessage. The bearer setup procedure as described in FIG. 10 would notneed to be changed and would incorporate the new information elementHARQ_flush_timer in the existing message such as RB_setup,RB_reconfiguration, RRC_connection setup, TrCH_reconfiguration,cell_update or a handover_command. If the HARQ timer value is notsignaled it may have a predetermined default value. Furthermore thegranularity of the timer values may increase with higher values of theparameter HARQ_flush_timer. For example when assuming possible timervalues of 5 ms, 10 ms, 15 ms, 20 ms, 30 ms, 50 ms or 100 ms, theHARQ_flush_timer could be represented by 3 bits.

Furthermore, the communication terminal may change the selection oftransmission parameters assuming it knows that the soft buffer hasalready been flushed in some or all Node Bs. There are for instancedifferent strategies for incremental redundancy depending on how manyretransmissions have already been sent. Some UEs send a low code rate atthe initial data packet to come close to the code rate needed fordecoding. In further retransmission data packets only little redundancyis added. If it is known that a buffer region associated with a datapacket and related retransmission data packets has been flushed in someor all Node Bs, the UE can again start from initial transmission. Thesame could apply for systems using different modulation constellationsin the different transmissions such as constellation rearrangement. (seefor example 3GPP TS 25.213: “Spreading and modulation (FDD)”, availableat http://.www.3gpp.org).

In FIG. 12 an exemplary HARQ transmitter operation is shown. The UE maybe ready for data transmission if the UE is synchronized, the radiobearer has been configured correctly by RRC etc. If the UE is allowed totransmit, has data in its transmission buffer, sufficient transmit poweretc. it may transmit one or more packets within a TTI as indicated byblock 1201. A packet is usually called Packet Data Unit and may be asegment of another packet such as an IP packet or may also be aconcatenation of multiple packets. In step 1202 the UE may select thetransmission parameter such as transport block size, modulation andcoding scheme, number of codes, power, constellation etc. and may sendthis side information or control information before or along with thepacket that is being transmitted in step 1203.

There are many alternatives how the feedback message is generated andprocessed. In this example all Node Bs send feedback and if one of theNode B sends an ACK (see step 1204) the packet is considered as beingcorrectly received and may be removed from the transmitter buffer (seestep 105). Next, for each Node By within the active set it may bechecked whether an acknowledgement for the transmitted packet has beenreceived in step 1206. In case a ACK has been received for a particularpacket X_(i) the corresponding timer T_(i,y) is stopped in step 1207. Ifno ACK from a base station has been received the process advances toblock 1209. The steps that may be executed by all Node Bs in the activeset (y=0,1 . . . Y) which is indicated in the figure by multiple shapes.This implies that there may be as many timers in the UE as there areNode Bs in the active set.

In case no ACK has been received for a the transmitted packet X_(i), thepacket may be scheduled for a retransmission in step 1208. The problemof non-synchronized soft buffer may occur when a new data packet istransmitted which is different from the one that is still stored in thesoft buffer, i.e. a previously transmitted data packet. Since the Node Bmay not know when is has missed a transmission, the timer is startedafter each reception.

In step 1210 the UE may start or restart the timers for every packet andeach Node B within the active set for which a negative acknowledgementNAK is received (see step 1209). If a Node B has missed the packet(neither ACK nor NAK is transmitted), the timer will not be affected andthe procedure advances to block 1211.

The timer may preferably be set to a multiple value of the TTI and willbe decremented each TTI in step 1211.

If a timer expires at a Node B (see step 1212), the UE may know that therespective timer expired at that particular Node B and the soft bufferfor that packet was flushed at that particular Node B (see step 1213).In the illustrated embodiment the UE may send a possible retransmission,i.e. an initial data packet in this case, with the initial parametersettings. If nothing has been scheduled within a TTI all timer will bedecremented as well by one TTI. The decision in the UE to restart withinitial transmission parameter may depend on other parameters orconditions such as at how many Node Bs the buffer was flushed, on howmany retransmissions have already been sent, on how much the channelconditions already changed in the meantime, etc.

It is further noted that only one selected Node B of the active set maysend a feedback message to the UE to indicate the reception state (ACK,NAK) of a transmitted data packet/retransmission data packet. In thelatter case, only one timer for each transmitted data packet may bemaintained. In this case it needs to be mentioned that it may not beensured that the soft buffer of the other Node Bs not sending feedbackare still always fully synchronized. In that situation it may bebeneficial to use the invention as outlined above or soft buffersynchronization by signaling among the active sets Node Bs, as outlinedin the copending application “Base Station synchronization in SoftHandover” (attorney's docket number EP28260), filed on the same date asthe present application. In the following other actions that may beperformed by the UE are outlined. These are mainly actions that avoidthat the Node B flushes the buffer region for a particular associateddata packet retransmission process. Therefore the UE timer has to be setto a value less then the Node B timer to trigger that action well intime, i.e. to be able to send e.g. a retransmission data packet arrivingat the Node B before time expiry.

If the UE is in a scheduled mode it needs to transmit capacity requestmessage to the Node B. These capacity request may contain differentattributes to support the Node B in making the scheduling decision. Suchparameters could be for instance the priority of the transmission,amount of data in the buffer as well as the time that is available untilthe packet must be transmitted. In another aspect of the invention theUE considers the timer for the generation of the UL capacity request aswell as for the setting of parameters which will be sent within thecapacity request.

It has been outlined that the timer may be calculated according to theworst case assuming continuous transmission of data packets from the UEand an immediate reuse of the same HARQ process and sequence number by anew data packet. This is in order to avoid erroneous combining of softbuffer values of a packet by all means. On the other hand such a casemay only rarely occur in reality and it is more likely that soft buffervalues are cleared from time to time although there may still be asomewhat delayed retransmission coming. In another embodiment of thepresent invention the timer may be set to a larger value than in thedescribed worst case scenario. In that case the UE has to preventerroneous combining in extreme situations close to the worst case byappropriate actions as outlined below.

Depending on the feedback scheme the UE is aware of some or all Node Bs'HARQ contexts i.e. state of the different processes or the ARQ window,the timer, the soft buffer consumption etc.. If the ACK/NAK is sent byall Node Bs the UE may know due to a missing ACKINAK that the Node B hasmost likely missed a certain packet. If this was an initial transmissionit is clear that the Node B's soft buffer has not been updated and ishence not synchronized to the buffers of the other Node Bs that havesent feedback. The UE may thus predict when an error situation canpotentially occur and avoid that case.

Since the error case only happens if a new packet is transmitted thetransmitter can use a different HARQ process and/or sequence number (orNDI) for the new packet and thus avoid a potentially wrong combining. Ifsoft buffer storage space is limited there must be a HARQ process and/orsequence number (or NDI) value available, which is not in the state ofprocessing or having values stored for combining. If the whole softbuffer is in use the respective retransmission process may be stalleduntil an ACK for another packet in the soft buffer is received and thatHARQ process and/or sequence number (NDI) may be reused. It may alsostall the HARQ process until the timer expires and restart with the samepacket. In general it should be avoided that the UE has to wait untilthe timer in the Node B expires in order to reduce latency.Nevertheless, there may still be special cases, depending on the datapacket retransmission scheme design, for which the expiry of the timeris desirable. Instead of waiting for the expiry of the timer in the NodeB the UE may itself initiate a partial or full of a soft buffer regionin the Node B. This may for instance be done by using e.g. a flushindicator, by forwarding the HARQ window artificially, which isequivalent to a flush of a part of the whole buffer or by simplyaborting some packets which have not been acknowledged.

If the UE is not able to retransmit packets, e.g. the transmit power isnot sufficient, scheduling the retransmission in time is not possible,etc., it may signal this to the Node B in order to avoid a flushing ofthe soft buffer. This could for instance be done by a flag along withthe other HARQ related control information such as HARQ process numberand sequence number (or NDI). A special restart request messagecomprising the flag may be sent by the UE to instruct a Node B torestart a particular running timer. Upon reception the Node B will stopor preferably restart the timer for that packet and will maintain thesoft buffer. Another possibility would be a kind of zero payload packetwith certain control information, but with a transport block size ofzero. That means that there is not real data transmitted. Although thistransmission will consume some resources it may be more radio efficientthen flushing the soft buffer which may have gathered almost enoughredundancy for successful decoding.

As outlined above, according to an embodiment of the present inventionit is desirable if a single Node B sends feedback messages to the UE toacknowledge a received data packet. Hence, the selection of the socalled serving Node B may require further considerations in order toprovide reliable feedback to the UE. Possible selection criteria relatedto radio link quality indicators, for the selection of a serving Node Bare outlined in the copending application “Serving Base Stationselection during Soft Handover” (attomey's docket number EP 28257),filed on the same date as the present application. The fact that a timerused for synchronization of soft buffer contents is near its expiry maybe interpreted as deterioration of uplink radio link conditions of thatparticular Node B. The signaling of this information to support servingNode B reselection depends on the UTRAN architecture that is considered.For the R991415 architecture, the information may be signaled from thecurrent serving Node B to the RNC. For the evolved architecture,however, radio-related protocol entities may be located in Node B+s. Itmay be up to the current serving Node B+ to select new serving Node B+and signal the decision to it. Therefore, in this case the fact that thetimer in the current serving Node B+ is near its expiry may not have tobe signaled to another network elements.

In the copending application the negotiation of activation time forserving Node B selection has been defined. A possible interaction withthe present application would be to consider the status of the timer forsoft buffer synchronization before proposing new activation time.Depending on the radio access network architecture, the actualdeployment, the transport technology etc. there may be different delayson lub/lur interfaces. Depending on these delays it may be beneficial touse the invention as outlined above or soft buffer synchronization bysignaling among the active sets Node Bs, as outlined in the copendingapplication “Base Station synchronization during Soft Handover”(attorney's docket number EP28260), filed on the same date as thepresent application. For short signaling delays within the network (e.g.all Node Bs part of the same cluster or Radio Network Subsystem) it maybe beneficial to use a synchronization method as described in thecopending application while for longer delays the present invention maybe preferred. Both method could also be applied in parallel anddepending. If the signaling is arrived the timer will be superseded orvice versa.

Another embodiment provides an alternative solution of the object of thepresent invention as stated above. If a packet is stored in the Node Band the Node B receives no retransmission for some time, it does notknow if it has missed the retransmission, which perhaps was receivedcorrectly by another Node B or if there was really nothing send in theuplink.

If there are no errors in the downlink feedback signalling the UE knowsits transmission state (HARQ context) of each Node B exactly. Even ifsome Node Bs missed some retransmissions completely, it will be known tothe UE based on the missing feedback of those UEs.

In this embodiment, an additional flag indicating whether to combine thereceived data packet with previous transmissions may be used. If apacket which is still stored in the soft buffer has not been scheduledin the meantime or has not been acknowledged by any of the Node Bs acombine indicator may be set by the UE to indicate to a Node B that thepacket can still be combined. This will give a Node B the guarantee thatthe packet can still be combined. For an initial transmission of a newdata packet the combine indicator may indicate that the received packetis not to be combined with previous data packets received and the Node Bmay flush a buffer region corresponding to the process number signalledwith the current received data packet. An advantageous combination withthe flush timer may also be possible. If the timer has expired and apacket is received with a not set combine indicator the packet will bediscarded.

If during a pending retransmission the channel conditions have beenchanged in the mean time it may also be beneficial to change thetransmission format of the packet. This implies that not combing will bepossible. In that case the flush bit could be set although the samepacket with the same sequence number is sent. The receiver may flush theHARQ buffer although a packet with the same sequence number (of processnumber and NDI) is still being buffered. One advantage compared toincrementing the sequence number is that there is no packet missing inthe reordering buffer.

For fast cell cite selection the problems arising are similar to thosein the introduction of this application. In contrast there may be nosoft handover for a UE, but a fast switch between different cells.

In a further embodiment of the present application the principlesunderlying the present invention as outlined above may be also appliedto HARQ soft buffer synchronisation during fast cell site selection(FCS). Using fast cell cite selection the UE always transmits to asingle cell preferable the cell with best channel characteristics or thelowest load (no soft handover transmission). Depending on the detailedprotocol the UE may switch between cells within a specific time or eachTTI. The cell switch may be done autonomously by the UE or may be fullyor partly controlled by the network. In the same way as in soft handoverthe soft buffer may need to be synchronised before the next transmissionmay arrive at the same Node B. For FCS the time available forsynchronisation may consider a switch from this cell and back to thiscell. The same further embodiment may apply if there are differentscheduling modes, whereas one mode may be characterized by the supportHARQ while the other may not. If the UE is in scheduled mode it may notsupport HARQ while it is in autonomous mode it may support suchfunctionality. One of the reasons for this may be that for scheduledmode more control information needs to be signalled between the UE andthe Node B. This could be combined with control information required forHARQ operation. When switching back and forth the soft buffer may besynchronised as well.

The RNC may not be aware of a mode switch or a cell switch performed byNode B and mobile terminal. As soon as the RNC, serving as a reorderingentity, receives packets from a new Node B it may inform the previous orall other Node Bs in the active set to flush their soft buffer.Alternatively the new Node B may know if a cell switch has beenperformed and can inform the old or the other Node Bs about it. Theother Node Bs may flush their buffer accordingly. The Node B which isaware of the mode or cell switch could also inform the RNC in Rel99/4/5architecture or the current Serving Node B+ in the evolved architectureabout this event. The RNC or the Serving Node B+ may also inform theother Node Bs of the active set to flush their buffer accordingly. Ifcell and mode selection is done on a slow basis and not packet perpacket the soft buffer may be synchronised before a cell or mode switchback to the previous cell or mode can happen.

As previously discussed there may be the drawback that soft buffervalues are flushed although retransmission may still arrive. If FCS ormode switches are done on a very fast basis e.g. per TTI there may be ahigh likelihood that a reselection of the same cell or mode occursfrequently. In that case it may be beneficial to keep the soft buffervalues for a potential switch back to the cell or mode for some periodof time. This will allow for a combining of retransmissions withprevious transmissions that are already buffered in the soft buffer. Itmay also happen that the communication terminal or mobile terminal doesnot transmit any data after the switch to a new cell or new mode. Inthat case it may be decided to switch back to the previous cell or modeand to continue transmission with the same state of the associatedbuffer region. The period of time until a flush will be done may againbe defined by a threshold time period whereas at least one of the basestations and the communication terminal may be monitoring the timeelapsed since the storage of said data packet in said associated bufferregion. The threshold time period may be calculated in a similar manneras described before maybe considering an additional cell or modeswitching time. There may again be a tradeoff between the minimum lengthof the threshold time period and the gain from soft combining. Erroneouscombining may be prevented by the described method or by a combinationwith other methods. The described threshold time period for FCS or amode switch may be the same or different from the threshold time periodfor the soft handover i.e. the plurality of base stations. If the valueis different it may also be signaled to at least one of the respectivebase stations by radio network control signaling and the communicationterminal via radio resource signaling in a similar manner as describedbefore.

Finally, it is noted that the present invention described above may beused for different types of RAN architectures. E.g. the presentinvention is applicable to the UTMS R99/415 UTRAN architecture asillustrated in FIG. 2 as well as the evolved UTRAN architecture asillustrated in FIG. 4.

1-39. (canceled)
 40. A method for receiving data in a data retransmission scheme in a mobile communication system comprising a communication terminal and a plurality of base stations, wherein said communication terminal is in communication with said plurality of base stations during a soft handover, the method comprising: receiving data from the communication terminal by at least one of the base stations, storing said received data in an associated soft buffer of said at least one of the base stations, decoding said received data at said at least one of the base stations, and using the time elapsed since storing said data in the associated soft buffer in order to flush the soft buffer.
 41. The method for receiving data according to claim 40, wherein the data is stored in the associated soft buffer in order to be combined with a retransmitted data if the received data is not decoded successfully.
 42. The method for receiving data according to claim 40 or 41, further comprising flushing the soft buffer based on the elapsed time.
 43. The method according to claim 42, wherein if said received data is decoded successfully, the soft buffer is flushed.
 44. The method according to claims 42, wherein the soft buffer is flushed, if the elapsed time is equal to or larger than a threshold time period.
 45. The method according to claims 43, wherein the soft buffer is flushed, if the elapsed time is equal to or larger than a threshold time period.
 46. The method according to claim 44, wherein the threshold time period is defined as a period after which a retransmission data can no longer be expected in the base station.
 47. The method according to claim 40, further comprising: receiving a retransmission data from the communication terminal by at least one of the base stations, storing said received retransmission data in an associated soft buffer, decoding said received retransmission data, and if the received retransmission data is not decoded successfully, restarting the elapsed time.
 48. The method according to claim 40, further comprising: receiving a retransmission data from the communication terminal by at least of the base stations, storing said received retransmission data in an associated soft buffer, decoding said received retransmission data, and if said received retransmission data is decoded successfully, stopping the elapsed time.
 49. The method according to claim 40, further comprising: combining a retransmission data with the previously received data to produce a combined data, and decoding the combined data.5
 50. The method according to claim 40, further comprising if the elapsed time is equal to or-larger than a threshold time period, stopping the elapsed time.
 51. The method according to claims 44, wherein said threshold time period is of configurable duration.
 52. The method according to claims 50, wherein said threshold time period is of configurable duration.
 53. The method according to claim 51, further comprising signaling the duration of said threshold time period to said at least one of the base stations by radio network control signaling from a control unit in the mobile communication network.
 54. The method according to claim 53, wherein the duration of said threshold time period is signaled to said at least one of the base stations in an information element of a NBAP message.
 55. The method according to claim 51, further comprising signaling the duration of said threshold time period to said communication terminal by radio resource control signaling from a control unit in the mobile communication network.
 56. The method according to claim 55, wherein the duration of said threshold time period is signaled to the communication terminal in an IE of at least one of a radio bearer setup message, a radio bearer reconfiguration message, a radio resource control connection setup message, a transport channel reconfiguration message, a cell update message, and a handover command message.
 57. The method according to claim 40, further comprising transmitting a message from at least one of the base stations to the communication terminal indicating whether said at least one of the base stations decoded said received data successively.
 58. The method according to claim 40, further comprising forwarding the received data to a control unit of the mobile communication system by one of said at least one of the base stations that did decode the received data successively.
 59. The method according to claim 40, further comprising receiving a capacity request message from said communication terminal by at least one of the base stations requesting additional transmission capacity for a retransmission data.
 60. The method according to claim 59, wherein said capacity request message comprises at least one of a transmission priority of data to be transmitted by said communication terminal, a size of data in a transmission buffer of said communication terminal, a duration of said elapsed time, an identification of data, or a channel for which capacity is requested.
 61. The method according to claim 59 or 60, further comprising transmitting a capacity grant message from said at least one of the base stations to said communication terminal, wherein the capacity grant message indicates a transmission capacity assigned to said communication terminal for data transmission.
 62. The method according to claim 40, wherein the data is received via a dedicated channel.
 63. The method according to claim 40, further comprising receiving a restart request message from said communication terminal by at least one of the base stations, wherein said restart request message indicates data for which the elapsed time has to be restarted.
 64. The method according to claim 63, wherein said restart request message comprises control information and no payload data or dummy payload data.
 65. The method according to claim 44, wherein said data retransmission scheme is a window-based data retransmission scheme, and the method further comprises calculating said threshold time period based on the time required for the transmission of all data within a window of the data retransmission scheme.
 66. The method according to claim 50, wherein said data retransmission scheme is a window-based data retransmission scheme, and the method further comprises calculating said threshold time period based on the time required for the transmission of all data within a window of the data retransmission scheme.
 67. The method according to claim 44, further comprising calculating said threshold time period based on the time interval between the reception of an initial data and the reception of a retransmission data.
 68. The method according to claim 50, further comprising calculating said threshold time period based on the time interval between the reception of an initial data and the reception of a retransmission data.
 69. The method according to claim 44, further comprising calculating the duration of said threshold time period based on the size of said soft buffer, a maximum number of retransmissions in the data retransmission scheme, a communication terminal's processing time for a feedback message, a respective base station's processing time for a received data and a transmission time interval.
 70. The method according to claim 50, further comprising calculating the duration of said threshold time period based on the size of said soft buffer, a maximum number of retransmissions in the data retransmission scheme, a communication terminal's processing time for a feedback message, a respective base station's processing time for a received data and a transmission time interval.
 71. A base station in a mobile communication system comprising a communication terminal and a plurality of base stations, wherein said base station comprises means for implementing the method according to claim
 40. 72. A base station for receiving data in a data retransmission scheme in a mobile communication system comprising a communication terminal and a plurality of base stations, wherein said communication terminal is in communication with said plurality of base stations during a soft handover, the base station comprising: a receiving section that receives data from the communication terminal, a soft buffer that stores said received data, and a decoding section that decodes said received data, wherein the base station is operable to use the time elapsed since storing said data in the associated soft buffer in order to flush the soft buffer.
 73. The base station according to claim 59, wherein the base station is operable to store the received data in the associated soft buffer in order to be combined with a retransmitted data if the received data is not decoded successfully.
 74. The base station according to claim 59 or 60, wherein the base station is operable to flush the soft buffer if said received data is decoded successfully.
 75. The base station according to claim 59, wherein the base station is operable to flush the soft buffer if the elapsed time is equal to or larger than a threshold time period.
 76. The base station according to claim 62, wherein the threshold time period is defined as a period after which a retransmission data can no longer be expected in the base station.
 77. The base station according to claim 59, wherein the receiving section receives a retransmission data from the communication terminal, the soft buffer stores said received retransmission data, and the decoding section decodes said received retransmission data, wherein if the received retransmission data is not decoded successfully, the base station is operable to restart the elapsed time.
 78. The base station according to claim 59, wherein the receiving section receives a retransmission data from the communication terminal, the soft buffer stores said received retransmission data, and the decoding section decodes said received retransmission data, wherein if said received transmission-data is decoded successfully, the base station is operable to stop the elapsed time.
 79. The base station according to claim 59, wherein the base station is operable to combine a retransmission data with the previously received data to produce a combined data, and the decoding section decodes the combined data.
 80. The base station according to claim 59, wherein the base station is operable to stop the elapsed time if the elapsed time is equal to or larger than a threshold time period. 